Water purification typically produces a first effluent of relatively “clean water” and a second effluent of “waste water” which include unwanted contaminants. The softening of hard water by the removal of calcium and magnesium is required for both industrial and household use. Known water softening processes proceed either by way of ion-exchange, membrane softening or precipitation. In the ion-exchange processes, the calcium and magnesium ions are exchanged for sodium and regeneration of the ion-exchange resin is achieved with a large excess of sodium chloride. This process creates a regeneration effluent that is a relatively concentrated aqueous solution of sodium, calcium and magnesium chlorides, which is normally discarded. Consequently, by this method, considerable amounts of sodium, calcium and magnesium salts in solution are lost.
Alternatively, it is possible to use weak acid resins which exchange hydrogen ions for calcium and magnesium, and to regenerate the spent resins with a mineral acid. While this method creates less waste water, it is more expensive and yields relatively acidic soft water which is corrosive.
Membrane softening concentrates the calcium, magnesium salts and salts of other divalent ions to produce waste waters which require costly disposal.
In the “lime-soda” precipitation process, lime is added to hard water to convert water-soluble calcium bicarbonate into water-insoluble calcium carbonate. This process results in waste water which is difficult to filter and requires cumbersome treatment.
Accordingly, it would be highly advantageous to provide improved methods of disposing of salty waste waters by methods which provide a beneficial use of the waters, which preserves the value of the salts.